The present invention relates to ultrasonic imaging with electronic scanning, enabling the exploration of a part or an organ to be analyzed, notably in a plan (C scan mode echography) or in depth (B mode echography).
Such an imaging device is capable of numerous applications, particularly in the field of medicine, in which field imaging by ultrasonic offers, over imaging by ionizing radiation (including X-rays), the great advantage of not subjecting the patient to harmful radiation.
Before defining the invention, it would seem desirable to recall some theoretical considerations which relate to the ultrasonic transmitters, although they apply just as well to receivers.
The energy of an ultrasonic beam transmitted to a medium by n elementary transducers distributed along a direction Ox may be focused at a point M in front of O and at the distance Y.sub.M from the straight line on which the n elementary transducers are distributed. If the radiation has a wave length .lambda. in the propagation medium concerned, each transducer of order i must receive a signal whose phase lead with respect to the transducer placed at 0 is: EQU .phi..sub.i = .pi. x.sup.2.sub.i / .lambda. y (1)
x.sub.i being the abscissa of the transducer with respect to 0.
In other words, the complex amplitude A*.sub.i of the signal applied to the elementary transducers of order i must be: EQU A.sup.*.sub.i = A.sub.o exp (j .phi..sub.i) (2)
whose real part is: EQU A.sub.o cos .phi..sub.i ( 3)
and imaginary part is: EQU A.sub.o sin .phi..sub.i ( 4)
There have been described ultrasonic imaging devices in which pseudo-focusing is achieved by energizing simultaneously and in phase a group of elementary transducers whose cumulative length along Ox is b = 2 .sqroot. Y.lambda., i.e. corresponds to the first zone of the Fresnel representation. Resolution of such a system is poor. It has also been suggested to associate a generator and a receptor to each elementary transducer and to simulate the phase repartition of several successive Fresnel zones. The same successive elementary transducers are used for reception and for transmission, with the same distribution, corresponding to a plurality of even and odd Fresnel zones associated with the focussing being sought. Such a system is quite complex and expensive. In addition, the main central lobe which provides the required focussing is accompanied by secondary lobes which may be unfavorable when the target is highly reflective. For example, in the case of heart imaging by echography B the rear wall of the heart provides specular reflection and often alters the visualization of targets reflecting less powerful echoes, which are located in the vicinity.
It is an object of the invention to provide an imaging device which overcomes the limitations of the prior art devices.
According to an aspect of the invention, there is provided a device for ultrasonic imaging with electronic scanning, having an array of N identical elementary transducers distributed at equal intervals along a scanning direction and operating at frequency f; a single signal generator for generating electrical signals; a single signal receiver for receiving and processing electrical signals; means for storing an electrical phase distribution signal representative of the phase distribution of n signals associated with n adjacent transducers of said array, n being an integer greater than 1 and less than N, each said signal having a frequency f and having any one of m phases with respect to the remaining said n signals (m being an integer less than n), said electrical phase distribution signal being chosen such that said n adjacent transducers focus energy at a point located at a predetermined distance from said array of transducers; switch means for simultaneously and temporarily applying said signal from said generator to a set of n transducers in accordance with said phase distribution and subsequently for simultaneously and temporarily applying the echo signals from a set of those n' transducers among the n transducers which are closest to said point to said receiver; and sequencing means for causing said switch means to apply said signal to n transducers at each of a plurality of successive times and to apply said echo signal to said receiver, each time to and from a set of n and n' transducers, respectively, which is shifted by at least one said transducer of said array of transducers, such that scanning occurs through said array.
m will preferably be equal to two; then some of the transducers will receive the energy with a reference phase and the others energy which is 180.degree. out of phase with the reference phase.
The number n' is advantageously chosen to correspond with the first Fresnel zone of the phase distribution ensuring focussing at the predetermined distance. In other words, the n' transducers connected for transmission (or reception) are the transducers which are located in the middle of the group of n transducers and which are connected to the receiver (or generator) with the same phase during reception (or transmission). Thus the secondary lobes upon the transmission are almost completely suppressed. With reception (or transmission) continuing to take place at full aperture, the level of the sidelobes is lowered to an acceptable threshold and the signal is "apodized" with a diminution of the resolution which is only very slight.
According to a particular embodiment of the invention, the storage means include two shift registers having p.n positions, which store, in operation:
the first, in positions 1, p, 2p, . . . , (n-1)p, the location of those of the n elementary transducers to be connected to the generator with the first phase; in the positions 2, p+1, . . . , (n-1)p+1, the location of those of the n elementary transducers to be connected to the receiver with the first phase for achieving focussing at a first predetermined distance from the line; and if necessary, in a group or other groups of n positions, the location of the n elementary transducers to be connected to the receiver with the said first phase for achieving focussing or focussings at another predetermined distance or at other predetermined distances;
the second, in positions 1, p, 2p, . . . , (n-1)p, the location of those of the n elementary transducers to be connected to the receiver with the second phase; in the positions 2, p+1, . . . , (n-1)p+1, the location of those of the n elementary transducers to be connected to the receiver with the second phase for achieving focussing at a second predetermined distance from the line; and if necessary, in a group or other groups of n positions, the location of the n elementary transducers to be connected to the receiver with the said second phase for achieving focussing or focussings at the other predetermined distance or at other predetermined distances; a clock being provided for causing the shifting or the contents of the said registers and their transfer in series into two shift registers of N positions, each position controlling the switching means associated with one elementary transducer,
said clock causing the shifting of the contents of the respective said registers of N positions at a rate such that after each emission the transducers are connected in accordance with successive reception distributions corresponding with the path durations for the focal distance chosen.
In accordance with another aspect of the invention, the switching means are designed to change the reception connections after each transmission at a time which has alternately one or other of two delays with respect to the emission and to block the receiver during a predetermined period .delta.t after each switching.
The invention will be better understood from reading the description which follows of devices which constitute particular embodiments.